Multi-year El Niño events tied to the North Pacific Oscillation

Multi-year El Niño events induce severe and persistent floods and droughts worldwide, with significant socioeconomic impacts, but the causes of their long-lasting behaviors are still not fully understood. Here we present a two-way feedback mechanism between the tropics and extratropics to argue that extratropical atmospheric variability associated with the North Pacific Oscillation (NPO) is a key source of multi-year El Niño events. The NPO during boreal winter can trigger a Central Pacific El Niño during the subsequent winter, which excites atmospheric teleconnections to the extratropics that re-energize the NPO variability, then re-triggers another El Niño event in the following winter, finally resulting in persistent El Niño-like states. Model experiments, with the NPO forcing assimilated to constrain atmospheric circulation, reproduce the observed connection between NPO forcing and the occurrence of multi-year El Niño events. Future projections of Coupled Model Intercomparison Project phases 5 and 6 models demonstrate that with enhanced NPO variability under future anthropogenic forcing, more frequent multi-year El Niño events should be expected. We conclude that properly accounting for the effects of the NPO on the evolution of El Niño events may improve multi-year El Niño prediction and projection.

In the last section, "Projections of future climate change", the authors notice that multiyear El Niño events have occurred more often during recent years and explore the possibility that these long-lasting events might become more frequent under anthropogenic forcing. Furthermore, they hypothesise that the increased frequency in multi-year events might be driven by enhanced NPO variance under climate change. This is an interesting hypothesis, and I appreciate their attempt find some evidence of this by looking at CMIP5/6 projection. However, I find this analysis quite preliminary and rather inconclusive. Even assuming all the El Niño events to be independent from each other, enhanced NPO (i.e., an ENSO precursor) variance would lead to more El Niño events, and thus the likelihood of occurrence of a multi-year event. This section is quite interesting and should be expanded a little. In addition to further analysis supporting your point, I suggest to add in figure 6, a subplot comparing the occurrences of multi-year events between period s? with low and high radiative forcing (e.g., 1960-1990 vs 2060-2090). Furthermore, I am a little surprised the authors did not mention the study of Liguori and Di Lorenzo (2018); it supports the authors' hypothesis of an enhanced NPO variance under anthropogenic forcing. Specifically, Liguori and Di Lorenzo analyse a large ensemble of climate simulations and found a robust increase in the PMM activity under anthropogenic forcing (i.e., RCP8.5). While they explained this increase with a theory involving the WES feedback, it is possible that the real cause is an increase in the NPO variance. Line 324-325: "Our study differs from previous studies by attributing the dominant source of multi-year El Niño to NPO-like extratropical". Perhaps, you should soften this statement as I don't think you present enough evidence for the NPO to be a dominant source. As you write in line 361 there are many other sources.

Reviewer #3 (Remarks to the Author):
Comments on the manuscript entitled "Multi-year El Niño events tied to the North Pacific Oscillation" by R. Ding et al. In this study the authors investigated the physical reasons for the multi-year El Niño events based on both data diagnoses and numerical experiments. It is found that the North Pacific oscillation (NPO) is a key source for such events. A two-way feedback mechanism between the tropical sea surface temperature (SST) anomalies and extratropical NPO was proposed to explain the long-lasting El Niños. This study in interesting and important in understanding the ENSO variability. However, there are major flaws existing in the present study. This manuscript might be accepted for publication after major revision. My specific comments on this manuscript are listed below.

Specific comments: 1. As revealed in previous and this studies, the NPO during the boreal winter can induce a central Pacific (CP) El Niño. However, in this study the Niño3.4 index was used in representing ENSO but not an index of CP El Niño. Why is not an index of CP El Niño used in this study? How about the results will be if a CP El Niño index is used?
The authors need to give explanations on these issues. 2. In the section of Introduction, the authors mentioned that the "NPO may also play an important (but not dominant) role in developing multi-year La Niña events through a similar mechanism" (Lines 9496). However, in the manuscript the multi-year La Niña events were almost not dealt with although they were listed in Supplementary Table 1. I suggest the authors give evidence for such statement. 3. It seems that the authors chose the multi-year El Niño events subjectively based on Supplementary Fig. 1. I suggest using an objective method to define and choose objectively the multi-year El Niño events. 4. It is hard to see that the SLP anomalies in Supplementary Figs. 3a-3e were similar to a typical NPO. The authors should not only show the NPO indices (Fig. 1a), but also calculate the spatial correlations between the SLP anomalies shown in Figs. 3a-3e and those associated with a typical NPO to illustrate how similar the SLP anomalies to those of the NPO. 5. Based on Supplementary Fig. 4, the authors concluded that "the NPO signatures preceding multi-year El Niño events were not accompanied simultaneously by large sea surface temperature (SST) anomalies in the tropical Pacific" (Lines 113-117). Such conclusion may not be correct. As seen in Supplementary Fig. 4 Supplementary Fig. 5 shows that the positive correlation of JFM(0) NPO index with the lagged Niño3.4 SST persisted through the following year until JFM(2). Such longlasting correlation should be effective for all El Niño events. Physical explanations should be given why some El Niño events are long-lasting ones but some not in the condition of the long-lasting effect of NPO. 7. Based on Fig. 2, the authors gave a conclusion that "positive SST anomalies in JFM(0) extended from the subtropical northeastern Pacific to the central equatorial Pacific and led to equatorial Pacific warming during the subsequent JFM(1)". However, as seen in Supplementary Fig. 6, in JAS(0) a weak warming occurred in the equatorial eastern Pacific and strengthened afterwards, which does not seem to be related to the SST anomalies in the subtropical northeastern Pacific but developed locally. The authors should provide evidence to prove if the positive SST anomalies in the central and eastern equatorial Pacific came from the subtropical northeastern Pacific or from local development. Supplementary Fig. 11 to see if the circulation anomalies associated with the negative SLP anomalies around Hawaii weakened the trade winds in the equatorial Pacific. 9. In this study the Pacific meridional mode (PMM) is mentioned. I suggest using the North Pacific meridional mode (NPMM) instead of PMM in the manuscript because in the South Pacific Ocean there exists another PMM, the South Pacific meridional mode (SPMM), which is also closely related with ENSO (Min et al., 2017, J. Climate, 30, 1705-1720). 10. The historical simulations of CMIP5/6 models were from 1900 to 1999, during which the three multi-year El Niño events of 1957/58/59, 1968/69/70, and 1986/87/88 were included. I suggest the authors compare the three events in simulations with those in observations to see how well the simulated ones in representing the observed ones. 11. In this study the role played by NPO in multi-year El Niño events were stressed. In fact, the effect from the subtropical southern Pacific should also take effect in the multiyear El Niño events. For example, the evolution of the long-lasting El Niño events in 2014/15/16 were affected by the system in the subtropical southern Pacific (Min et   In addition, to check how well the preceding JFM(0) SLP patterns of the five multi-year El Niño events match the NPO typical pattern, we have calculated the spatial correlation coefficients of the JFM(0) SLP pattern with the typical NPO pattern ( Figure A1f; please also see our response to Reviewer #3 Specific comments #4). The spatial correlation coefficients for the five multi-year El Niño events are all significant above the 95% significance level ( Figure A1h). The spatial correlation coefficient for the 1986 map is as high as 0.67. These results support our argument that the North Pacific SLP precursor patterns for multi-year El Niño events are similar to a typical NPO pattern. computing the t-value and degrees of freedom for paired t-test is:

I suggest adding surface winds in all the figures showing SLP anomalies in
where S1 and S2 are the variances of variables X1 (the NPO index of multi-year El Niño events) and X2 (the NPO index of all winters), respectively, and n1 and n2 are the numbers of variables X1 and X2, respectively. According to the above equation, we can obtain: According to the table of t-distribution table, we can obtain the t value significant at the 95% level. S1, S2, n1, and n2 are known in advance. Therefore, according to the above equation, we can obtain the 95% confidence level of the average NPO indices over the five events (i.e., the error bar).
We would like to clarify that we only provide the statistical significance of the average NPO index (i.e., error bars) for the five events, but not for the JFM NPO index above 1. The horizontal dashed line in Figure 1a of the main manuscript simply shows that the JFM(0) NPO index for all 5 multi-year El Niño events exceeds 1.0 standard deviation.

109: was it the monthly mean NPO index that was normalized of the FMA NPO index?
Response: Yes, we first obtain the time series of the JFM (0)   Response: Thanks for the insightful comments from the reviewer. We agree with the reviewer that there are some inaccuracies with our statements here. The sentence "these NPO signatures preceding multi-year El Niño events are not accompanied simultaneously by large sea surface temperature (SST) anomalies in the tropical Pacific, implying that they may originate primarily from intrinsic atmospheric variability in the North Pacific rather than from tropical SST forcing" has been removed from the revised manuscript. Instead, we have added the following sentences in the section "Summary and Discussion" of the revised manuscript (please see lines 331-341): "While the North Pacific precursor patterns for multi-year El Niño events have a strong NPO/NPMM structure, we note that in observations there is also an evident La Niña-like condition in the precursor patterns (Fig. 2a). Given the few realizations in the observational record, the dynamical significance of the tropical Pacific cooling is still unclear. In the CMIP5/6 models, the probability of NPO events co-occurring with La Niña prior to multi-year El Niño events is relatively low (~10%; Fig. 1e). The multi-model ensemble precursor patterns also exhibit a weak cooling in the tropical Pacific, but its amplitude is much smaller compared to the strength of the NPO signal ( Supplementary Fig. 15a

Response:
We are grateful for the reviewer's helpful comments. Former Supplementary Figure 5 shows that the correlations between the NPO and Niño3.4 indices remain positive through the following year until JFM (2), suggesting that the NPO-related El Niño state may persist for more than two years. Based on these persistent positive correlations, we argue that the NPO tends to induce a slower phase transition of El Niño, thereby creating favorable conditions for the occurrence of multi-year El Niño events.
However, as the reviewer mentioned, although the correlations between the NPO and Niño3.4 indices remain positive, they are no longer statistically significant from JJA(1). We speculated that there are two reasons why the correlations between the NPO and Niño3.4 indices are no longer significant after JJA(1).
Firstly, the NPO is likely to be followed by not only multi-year El Niño but also single-year El Niño. Secondly, the number of multiyear El Niño events preceded by the NPO is limited in the observations. In the light of the reviewer's comments, we decided to remove the former Supplementary Figure    We have added a discussion of the tropical cooling in the section "Summary and Discussion" of the revised manuscript (please see lines 331-341): "While the North Pacific precursor patterns for multi-year El Niño events have a strong NPO/NPMM structure, we note that in observations there is also an evident La Niña-like condition in the precursor patterns (Fig. 2a). Given the few realizations in the observational record, the dynamical significance of the tropical Pacific cooling is still unclear. In the CMIP5/6 models, the probability of NPO events co-occurring with La Niña prior to multi-year El Niño events is relatively low (~10%; Fig. 1e). The multi-model ensemble precursor patterns also exhibit a weak cooling in the tropical Pacific, but its amplitude is much smaller compared to the strength of the NPO signal ( Supplementary Fig. 15a) El Niño events and the change in the NPO variance (see Figure B1c below), but also the inter-model relationship between the change in the ratio of the number of multi-year El Niño events to the total number of El Niño events and the change in the NPO variance (see Figure B1d below). We note that models with the increased variance of the NPO index in the future climate systematically produce a more frequent occurrence of multi-year El Niño events, and this tendency is statistically significant. Furthermore, the ratio of the number of multi-year El In addition to further analysis supporting your point, I suggest to add in figure 6, a subplot comparing the occurrences of multi-year events between period s? with low and high radiative forcing (e.g., 1960-1990 vs 2060-2090).

Response:
We appreciate the reviewer's suggestions. In the light of the reviewer's suggestions, we have now added a subplot in the former Figure 6 comparing the number of multi-year El Niño events that occurred in the present  and future  climates using the 23 selected CMIP5/6 models (see Figure B1a below). The present and future warmer climates are represented by the historical (HIST) and Representative Concentration Pathway 8.5 (RCP8.5) experiments, respectively. 16 of the 23 models (70%) simulate an increased frequency of multi-year El Niño events in the future warmer climate, with a good inter-model consensus (see Figure B1a below). The ensemble mean increase is 43% (from about 7 events per 100 years in the present climate to 10 events per 100 years in the future climate), which is significant above the 95% confidence level according to a bootstrap test ( Figure B1b).

Furthermore, I am a little surprised the authors did not mention the study of
Liguori and Di Lorenzo (2018)  The change in the number of multi-year El Niño events is defined as Δ=(m2-m1)/m1, where m1 and m2 represent the numbers of multi-year El Niño events that occurred in the present and future climates, respectively. The change in the ratio of the number of multi-year El Niño events to the total number of El Niño events is defined as Δ=m2/n2-m1/n1, where n1 and n2 represent the total number of El Niño events that occurred in the present and future climates, respectively.

II-B: Response to minor comments
1. Figure 4 and Figure B2 below). These results support our view that although the NPO acts as a precursor for ENSO, multi-year El Niño events are more closely tied to the precedent NPO than single-year El Niño events. Figure B2   Response: We would like to thank the reviewer for his/her insightful and constructive comments and suggestions for the manuscript. The manuscript has been revised as per the comments given by the reviewer, and please see our point-by-point responses (in blue) to all the comments below.

As revealed in previous and this studies, the NPO during the boreal winter can induce a central Pacific (CP) El Niño. However, in this study the Niño3.4 index was used in representing ENSO but not an index of CP El Niño. Why is not an index of CP El Niño used in this study? How about the results will be if a CP El
Niño index is used? The authors need to give explanations on these issues.

Response:
Thanks for the constructive comments from the reviewer. According to the schematic diagram in Figure 4 of the manuscript, the boreal winter NPO induces a CP-type El Niño event over the equatorial Pacific during the subsequent winter through its effect on the NPMM. This CP-type El Niño in turn feeds back into the North Pacific to force changes in atmospheric circulation over the Hawaiian region, which re-activate the NPMM to favor the development of another CP-type or EP-type El Niño event. In this two-way feedback process between the tropics and extratropics, the El Niño event of the first year is usually a CP type, but the El Niño event of the second year is not necessarily a CP type, and it could be an EP type. Therefore, if a CP El Niño index was used to measure the evolutions of multi-year El Niño events, it cannot well capture the temporal evolution characteristics of multi-year El Niño events. Taking the 2014/15/16 event as an example, the El Niño event during JFM of 2016 emerges as an EP type, and the CP El Niño index cannot capture the realistic intensity of this 2015/2016 El Niño event (see Figure C1 below). In contrast, as a commonly used index to define ENSO events, the Niño34 index is closely related to both indices of CP-type and EP-type ENSO, and was therefore used to represent ENSO in our study. The CP El Niño and Niño34 indices are provided in the top right. Secondly, the composite analysis of multi-year La Niña events preceded by the negative NPO event alone in CMIP5/6 models shows that a negative NPO event alone during JFM(0) can also lead to multi-year La Niña events through the two-way feedback mechanism between the tropics and extratropics. This suggests that although the phase transitions of La Niña are determined largely by the recharge process of equatorial oceanic heat content, the NPO may also play a role in developing multi-year La Niña events, resembling its role in developing multi-year El Niño events.

In the section of Introduction, the authors mentioned that the "NPO may also play an important (but not dominant) role in developing multi-year La
Based on the above two pieces of evidence, we argue that the NPO may also play an important (but not dominant) role in developing multi-year La Niña events.
We agree with the reviewer that the current analysis of multi-year La Niña events is preliminary. We plan to conduct further systematic and in-depth investigations of multi-year La Niña events in future work. In the light of the reviewer's comments, we have removed the following sentence from the section "Introduction" of the revised manuscript: "In addition, we argue that the NPO may also play an important ( (1)  "While the North Pacific precursor patterns for multi-year El Niño events have a strong NPO/NPMM structure, we note that in observations there is also an evident La Niña-like condition in the precursor patterns (Fig. 2a). Given the few realizations in the observational record, the dynamical significance of the tropical Pacific cooling is still unclear. In the CMIP5/6 models, the probability of NPO events co-occurring with La Niña prior to multi-year El Niño events is relatively low (~10%; Fig. 1e). The multi-model ensemble precursor patterns also exhibit a weak cooling in the tropical Pacific, but its amplitude is much smaller compared to the strength of the NPO signal ( Supplementary Fig. 15a). In this regard, further modelling studies are required to explore the relative importance of the NPO and La Niña states in developing subsequent multi-year El Niño events." 6. Supplementary Fig. 5 shows that the positive correlation of JFM (0) Figure 4 of the manuscript), both the NPO and its induced CP El Niño are necessary for generating multi-year El Niño events. If the JFM(0) NPO is followed by a CP El Niño in the following JFM(1), this CP El Niño will likely re-intensify in the subsequent winter through the two-way feedback mechanism between the tropics and extratropics, lasting for two years or longer and finally resulting in multi-year El Niño events. However, previous studies have suggested that the NPO tends to induce the CP-type El Niño, but not in all cases (Ding et al. 2015). Equatorial ocean dynamics, such as zonal advection in the tropical Pacific, can extend NPO-induced warming in the central equatorial Pacific eastwards, leading to EP El Niño events (Pegion et al. 2013). If the JFM(0) NPO is followed by an EP El 7. Based on Fig. 2, the authors gave a conclusion that "positive SST anomalies in JFM (0)   , which is also closely related with ENSO (Min et al., 2017, J. Climate, 30, 1705-1720.   (1), which often re-intensifies in the subsequent winter and finally develops into a multi-year El Niño event (please see Supplementary Figure 15 of the revised manuscript).

Response
The present study focuses on establishing the evidence that NPO dynamics act as a driver for multi-year El Niño events. The historical simulations of CMIP5/6 models provide additional evidence for the dynamic link between the NPO and multi-year El Niño and thus support the main findings from the observation data.
A more detailed comparison of multi-year El Niño events in CMIP5/6 models and observations is not the focus of the present study. We fully agree with the reviewer that it is very necessary to evaluate the performance of CMIP5/6 models in simulating multi-year El Niño events. In the future, we will conduct a detailed and in-depth assessment of the CMIP5/6 model's performance in simulating multi-year ENSO events as suggested by the reviewer.
11. In this study the role played by NPO in multi-year El Niño events were stressed.
In fact, the effect from the subtropical southern Pacific should also take effect in the multiyear El Niño events. For example, the evolution of the long-lasting El Niño events in 2014/15/16 were affected by the system in the subtropical southern Pacific (Min et al., 2015, Geophys. Res. Lett., 42, 6762-6770;Su et al., 2018, J. Climate, 31, 877-893). I suggest adding necessary discussions in the manuscript by referring these previous studies.
Response : We fully agree with the reviewer that forcing from the subtropical South Pacific could also have an impact on the evolutions of multi-year El Niño events. Following the reviewer's suggestion, we have added the following